The present invention relates to a motor controller for realizing a stable output torque in a wide range of rotational speeds of a motor used for an electric vehicle or the like subjected to field-weakening control with high efficiency.
In a brushless motor having no physical commutation mechanism, the stator current flowing in the stator windings is controlled in synchronism with the rotational position of the rotor and a predetermined output torque is generated by rotating the rotor in a predetermined direction. The brushless motor which forms a magnetic field by a permanent magnet is high in efficiency and is widely used for electric vehicles.
Now, an example of the prior art will be explained after describing the features and the field-weakening control of the brushless motor.
Features of brushless motor!
A permanent magnet is arranged on the rotor of the brushless motor thereby to form a magnetic field. The same direction as the magnetic field is called a d-axis, and the direction displaced by 90.degree. electrical angle from the d-axis as a q-axis. The q-axis is in the same direction as the counter electromotive force excited in the stator winding by the magnetic field. The current phase is advanced by adding a current in the direction of d-axis. As shown in FIG. 20, the magnitude of current is designated as I, the current component in the direction along d-axis as a d-axis current Id, the current component flowing q-axis as a q-axis current Iq, and the current phase as .beta..
When a current is supplied to the stator windings, the q-axis component Vq and the d-axis component Vd of the terminal voltage are expressed by equation 1. Also, the output torque Trq of the brushless motor is expressed by equation 2. In these equations, .omega.e is the electrical angular speed, R the resistance of the stator windings, .psi. the flux linkage of the magnetic field, Ld the d-axis inductance, and Lq the q-axis inductance. ##EQU1##
In a Surface Permanent Magnet motor (SPM motor) with permanent magnet arranged on the surface thereof, the d-axis inductance Ld is equal to the q-axis inductance Lq (Ld=Lq). Therefore, the second term of equation 2 is 0. As long as the magnitude I of the current is constant, the output torque Trq becomes maximum when the current phase .beta. is 0.degree.. With an Interior Permanent Magnet motor (IPM motor) with permanent magnet buried in the rotor, on the other hand, the d-axis inductance Ld is smaller than the q-axis inductance Lq (Ld&lt;Lq). The first term of equation 2, therefore, assumes a maximum value when the current phase .beta. is 0.degree., and the second term assumes a maximum value when .beta.=45.degree.. Consequently, as shown in FIG. 21, the output torque assumes a maximum value T0 for a certain value (.beta.0.degree.) of .beta. between 0.degree. and 45.degree..
Field-weakening control!
Assume that the brushless motor is in dynamic mode. As shown in the vector diagram (FIG. 22A) for field-weakening control, an induced voltage .omega.e.multidot..psi. increases with the increase in the rotational speed .omega. of the brushless motor. Once a voltage value V which is a vectorial sum of .omega.e.multidot..psi., R.multidot.Iq and .omega..multidot.Lq.multidot.Iq reaches a voltage-limit circle, the brushless motor cannot increase the rotational speed any more than .omega. associated with the voltage value V that has reached the voltage-limit circle.
In the case where the power supply is a battery, the terminal voltage and the current value of the battery undergo a change with the degeneration of the battery. For simplicity's sake, however, the terminal voltage of the battery (radius of the voltage-limit circle) is assumed to be constant.
Now, assume that the rotational speed of the brushless motor is to be increased. As shown in FIG. 22, the voltage .omega..multidot.Ld.multidot.Id in the direction returning to the interior of the voltage-limit circle is generated by supplying Id. As a result, a voltage margin is generated for increasing the rotational speed of the brushless motor (FIG. 22B). In the case where the rotational speed of the brushless motor is constant, the q-axis current Iq can be supplied by an amount equivalent to the voltage margin thus generated, so that an additional output torque can be generated by the brushless motor. In the case where the current value is constant, on the other hand, the rotational speed .omega. can be increased by an amount equivalent to the voltage margin generated (FIG. 22C). As described above, the control operation in which the d-axis current Id is supplied to the stator windings to generate a voltage margin is called the field-weakening control.
Assume that the current phase is changed while maintaining a constant current magnitude. The d-axis current Id increases, so that a voltage margin is generated thereby to produce the effect of the field-weakening control. At the same time, the q-axis current Iq, and hence, Lq.multidot.Iq is reduced, resulting in a smaller voltage value V thereby to generate an additional voltage margin.
The d-axis current Id for generating a voltage margin can be a minimum current required for the terminal voltage supplied to the brushless motor to return into the voltage-limit circle. In the case where the d-axis current Id more than the required minimum flows, the copper-loss increases and the efficiency of the brushless motor is adversely affected.
As far as the voltage has a margin, the stator current faithfully follows the stator current command as shown in FIG. 7A. Once the voltage margin is lost, however, the stator current cannot follow the stator current command as shown in FIG. 7B.
Example of prior art!
A conventional field-weakening control system for the brushless motor is described in Industrial Power and Electricity Application Research Society Materials IEA-92-30, Institute of Electrical Engineers of Japan.
The d-axis current command Id* is calculated according to equation 3 thereby to effect the field-weakening control. ##EQU2## where wbase is a basic rotational speed, .omega.max is a maximum rotational speed, and Id*max is a d-axis current associated with the maximum rotational speed .omega..
Also, Collection of Lectures No.74, pp.310-315, at General Assembly of Industrial Application Section, Institute of Electrical Engineers of Japan, held in 1991, discloses a method of field-weakening control, in which the d-axis current command Id* is calculated using a target rotational speed, a d-axis winding reactance, a q-axis winding reactance, a stator winding resistance, and no-load induced voltage at unit speed, etc.
In the actual brushless motor, however, the motor constants change with the change in resistance value according to the operating conditions, the change in inductance due to saturation of magnetic fluxes and secular variations. Therefore, the d-axis current command Id* calculated from the above-mentioned equation for the above-mentioned conventional art is not optimum for field-weakening control.
In the motor controller described in U.S. Pat. No. 5,652,495, a current error constituting the difference between the stator current command and the stator current is detected, and the d-axis current command Id* is increased when the current error is large, while the d-axis current command Id* is reduced when the current error is small, thereby to effect feedback control. Then, a required minimum of d-axis current command Id* can always be realized, and the increase in copper-loss due to the increased d-axis current Id is minimized. As a result, it is possible to converge to an operating point for field-weakening control at real time with high efficiency.
In the case of using the above-mentioned feedback control based on a current error, a gain indicating the rate of changing the d-axis current command Id* and a reference value indicating a reference current error are designed with a margin in order to secure stable operation in all the operating areas. As a result, an application over a wide rotational speed area makes it impossible to secure an optimum gain and an optimum reference value in a part of the operating areas. Thus, it sometimes occurs that an optimum response and an optimum output torque cannot be secured.
An object of the present invention is to further improve the technique described in U.S. Pat. No. 5,652,495, and in particular to realize a safe, stable driving operation of an electric vehicle by generating a smooth torque not in conflict with an operation command under any operating conditions.